For centuries we've been told the best way to know if you're a healthy weight is to calculate your body mass index (BMI), by dividing your weight in kilos by your height in metres squared.

All NHS doctors and nurses use it, but it's a system that was devised back in the 1800s when doctors still used blood-sucking leeches to get rid of infection – so is it still really the best indicator of health?

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Not according to a growing number of experts who argue that it doesn't take into account where you carry weight, which is the most significant factor in terms of future-proofing your health.

A BMI of 18-25 is considered healthy, 25-30 is overweight and 30-plus is obese, but many experts argue that tracking your waist size is a far more meaningful reflection of overall health.

"BMI takes no account of fat distribution," says Margaret Ashwell, a nutritional consultant and former government adviser. "Measuring your waist circumference, on the other hand, gives an indication of the amount of fat around your heart, liver, pancreas and kidneys."

"An apple-shaped woman with a BMI of 28 is at higher risk of type 2 diabetes and cardiovascular disease than a pear-shaped woman with an identical BMI," agrees Dr Dawn Harper. "That's because fat around your bottom and thighs is a store of energy that your body can call on in times of need. But fat around your middle produces chemicals that increase your health risks."

Several studies back this up, with one piece of research suggesting having a small waist and shapely bottom can increase your life expectancy by up to nine and a half years, compared to someone with a rounded tum. Many experts also say that a one-measurement-fits-all approach like BMI simply doesn't work.

"Black, Asian and certain other ethnic groups have an increased risk of diabetes and heart disease," says GP Dr Gio Miletto. So their BMI threshold should be lower to reflect that. Also, your BMI doesn't tell you very much at all about general health – you could be a skinny smoker living off baked beans and still have
a 'normal' BMI."

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Even world-class athletes would come a cropper with BMI. Chris Hoy, one of the most successful Olympians ever, with six Olympic gold medals plus 11 world- championship titles, would be classed as 'overweight' with a BMI of 27, and Ellie Simmonds, four-time Paralympic Champion and current holder of 13 world records would be teetering on 'obese' based on her BMI.

Dr Ian Beasley, a sports physician at The London Independent Hospital, who's worked with everyone from Team GB athletes to world-class footballers, can understand why. "BMI doesn't take into account where the fat is – or whether it's there at all. Loads of my athletes would come up as overweight based on BMI, because muscle is much denser than fat." He recommends focusing on how your body looks and feels when considering your weight:

"If you're carrying weight around your tummy, the best way to shed that is a combination of aerobic exercise (you don't have to go at it full pelt – on a scale of one to 10 aim for around six or seven) and a healthy diet. So cut down on calories and limit your carbohydrates and refined sugars."

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Sally Davies, chief medical officer at Public Health England agrees that BMI has its limitations: "It doesn't fully adjust for the effects of heights or body shape, which may be particularly important when comparing figures across ethnic groups," she says. But, unfortunately, there are currently no plans to introduce a different measurement.

"BMI has been widely used around the world for some time, enabling comparisons between areas, across populations and over time. It's an easy, cheap and non-invasive means of assessing excess body fat. Other measures, such as waist or hip circumference, body-fat ratio and skin-fold thickness may provide a better indication of 'fatness' but they are more difficult and expensive to collect in large numbers. Furthermore, the precise thresholds used to classify individuals as obese, overweight or underweight using those methods are not as well established. Although it's not a 'gold standard' measure of overweight or obesity, BMI's advantages make it currently the only viable option for producing high-level summary figures at population level."

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But although the NHS isn't planning on binning BMI anytime soon, experts agree that a waist-to height measurement is the most accurate and simplest way for you to keep track of your own health status.

Margaret studied data for thousands of people in the UK and found that those with the lowest waist-to-height ratios had the lowest death rates, and those with the highest waist-to-height ratios had – you guessed it – the highest risk, so it was a much more accurate measure than BMI.

So what should you do? "Measure your waist each week and try to keep it to less than half your height," says Margaret. So a woman who's 5ft 6in (66 inches) should try to keep her waist at 33 inches or smaller. But make sure you measure the right spot: "Stand up straight and measure halfway between your hip bones and the bottom of your ribs," says Dr Harper. "No breathing in – and no cheating!"

A better way to measure

Waist circumference

35+ inches for a woman suggests an increased health risk (40+ inches for a man).

Waist-to-height ratio

Aim to keep your waist less than half your height, so if you're 5ft 6in (66 inches) your waist should be 33 inches or smaller.

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